Update Regex to handle DTD markup declarations

[kraftner]

Fixes #8

[sindresorhus]

So not to make the regex totally unreadable, I think it would be better to just strip any [<!ENTITY [\s\S]*>] from the string before doing the existing regex check.

[kraftner]

Looking at the Wikipedia Article linked in the corresponding issue I have the impression that there are quite some more possible values than just <!ENTITY ... > this is why I kept it as general as possible.